[SYCL] Test functionality of USM pointers inside a struct

[elizabethandrews]

When working on PR2413, which deals with unused array of pointers inside a struct in user code, the question came up if everything works as it should for USM pointers/array of USM pointers inside a struct. (#2413 (comment))

Filing this issue to investigate if we have tests for this and if everything is working as it should be.

[elizabethandrews]

Tests checking USM functionality were added by @Fznamznon - intel/llvm-test-suite#1288